(Note: "m1" means make 1 -- cast on a new stitch by whatever means you prefer, either casting on with the backward loop or picking up the thread between two existing stitches and knitting into that.)

Make thumb gusset:
Round 1: K3, place marker, m1, K2, m1, place marker, knit around (4 sts between markers).
Round 2: K around.
Repeat rounds 1 and 2, increasing one stitch after the first stitch marker and one stitch before the second stitch marker on every odd-numbered row until you have 10 (12) (14) sts between markers.
Knit one round after last increases.
Remove thumb stitches as follows: K3, place 10 (12) (14) sts between markers on a holder or a strand of yarn, cast on 2, continue around.
You now have 30 (36) (40) sts on your needles.
K 16 (19) (20) rounds.